firewire: net: throttle TX queue before running out of tlabels

This prevents firewire-net from submitting write requests in fast
succession until failure due to all 64 transaction labels were used up
for unfinished split transactions.  The netif_stop/wake_queue API is
used for this purpose.

Without this stop/wake mechanism, datagrams were simply lost whenever
the tlabel pool was exhausted.  Plus, tlabel exhaustion by firewire-net
also prevented other unrelated outbound transactions to be initiated.

The chosen queue depth was checked by me to hit the maximum possible
throughput with an OS X peer whose receive DMA is good enough to never
reject requests due to busy inbound request FIFO.  Current Linux peers
show a mixed picture of -5%...+15% change in bandwidth; their current
bottleneck are RCODE_BUSY situations (fewer or more, depending on TX
queue depth) due to too small AR buffer in firewire-ohci.

Maxim Levitsky tested this change with similar watermarks with a Linux
peer and some pending firewire-ohci improvements that address the
RCODE_BUSY problem and confirmed that these TX queue limits are good.

Note:  This removes some netif_wake_queue from reception code paths.
They were apparently copy&paste artefacts from a nonsensical
netif_wake_queue use in the older eth1394 driver.  This belongs only
into the transmit path.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Tested-by: Maxim Levitsky <maximlevitsky@gmail.com>

drivers/firewire/net.c

@@ -27,8 +27,14 @@
 #include <asm/unaligned.h>
 #include <net/arp.h>
 
-#define FWNET_MAX_FRAGMENTS	25	/* arbitrary limit */
-#define FWNET_ISO_PAGE_COUNT	(PAGE_SIZE < 16 * 1024 ? 4 : 2)
+/* rx limits */
+#define FWNET_MAX_FRAGMENTS		30 /* arbitrary, > TX queue depth */
+#define FWNET_ISO_PAGE_COUNT		(PAGE_SIZE < 16*1024 ? 4 : 2)
+
+/* tx limits */
+#define FWNET_MAX_QUEUED_DATAGRAMS	20 /* < 64 = number of tlabels */
+#define FWNET_MIN_QUEUED_DATAGRAMS	10 /* should keep AT DMA busy enough */
+#define FWNET_TX_QUEUE_LEN		FWNET_MAX_QUEUED_DATAGRAMS /* ? */
 
 #define IEEE1394_BROADCAST_CHANNEL	31
 #define IEEE1394_ALL_NODES		(0xffc0 | 0x003f)
@@ -640,8 +646,6 @@ static int fwnet_finish_incoming_packet(struct net_device *net,
 		net->stats.rx_packets++;
 		net->stats.rx_bytes += skb->len;
 	}
-	if (netif_queue_stopped(net))
-		netif_wake_queue(net);
 
 	return 0;
 
@@ -650,8 +654,6 @@ static int fwnet_finish_incoming_packet(struct net_device *net,
 	net->stats.rx_dropped++;
 
 	dev_kfree_skb_any(skb);
-	if (netif_queue_stopped(net))
-		netif_wake_queue(net);
 
 	return -ENOENT;
 }
@@ -783,15 +785,10 @@ static int fwnet_incoming_packet(struct fwnet_device *dev, __be32 *buf, int len,
 	 * Datagram is not complete, we're done for the
 	 * moment.
 	 */
-	spin_unlock_irqrestore(&dev->lock, flags);
-
-	return 0;
+	retval = 0;
  fail:
 	spin_unlock_irqrestore(&dev->lock, flags);
 
-	if (netif_queue_stopped(net))
-		netif_wake_queue(net);
-
 	return retval;
 }
 
@@ -891,6 +888,13 @@ static void fwnet_free_ptask(struct fwnet_packet_task *ptask)
 	kmem_cache_free(fwnet_packet_task_cache, ptask);
 }
 
+/* Caller must hold dev->lock. */
+static void dec_queued_datagrams(struct fwnet_device *dev)
+{
+	if (--dev->queued_datagrams == FWNET_MIN_QUEUED_DATAGRAMS)
+		netif_wake_queue(dev->netdev);
+}
+
 static int fwnet_send_packet(struct fwnet_packet_task *ptask);
 
 static void fwnet_transmit_packet_done(struct fwnet_packet_task *ptask)
@@ -907,7 +911,7 @@ static void fwnet_transmit_packet_done(struct fwnet_packet_task *ptask)
 	/* Check whether we or the networking TX soft-IRQ is last user. */
 	free = (ptask->outstanding_pkts == 0 && ptask->enqueued);
 	if (free)
-		dev->queued_datagrams--;
+		dec_queued_datagrams(dev);
 
 	if (ptask->outstanding_pkts == 0) {
 		dev->netdev->stats.tx_packets++;
@@ -978,7 +982,7 @@ static void fwnet_transmit_packet_failed(struct fwnet_packet_task *ptask)
 	/* Check whether we or the networking TX soft-IRQ is last user. */
 	free = ptask->enqueued;
 	if (free)
-		dev->queued_datagrams--;
+		dec_queued_datagrams(dev);
 
 	dev->netdev->stats.tx_dropped++;
 	dev->netdev->stats.tx_errors++;
@@ -1063,7 +1067,7 @@ static int fwnet_send_packet(struct fwnet_packet_task *ptask)
 		if (!free)
 			ptask->enqueued = true;
 		else
-			dev->queued_datagrams--;
+			dec_queued_datagrams(dev);
 
 		spin_unlock_irqrestore(&dev->lock, flags);
 
@@ -1082,7 +1086,7 @@ static int fwnet_send_packet(struct fwnet_packet_task *ptask)
 	if (!free)
 		ptask->enqueued = true;
 	else
-		dev->queued_datagrams--;
+		dec_queued_datagrams(dev);
 
 	spin_unlock_irqrestore(&dev->lock, flags);
 
@@ -1248,6 +1252,15 @@ static netdev_tx_t fwnet_tx(struct sk_buff *skb, struct net_device *net)
 	struct fwnet_peer *peer;
 	unsigned long flags;
 
+	spin_lock_irqsave(&dev->lock, flags);
+
+	/* Can this happen? */
+	if (netif_queue_stopped(dev->netdev)) {
+		spin_unlock_irqrestore(&dev->lock, flags);
+
+		return NETDEV_TX_BUSY;
+	}
+
 	ptask = kmem_cache_alloc(fwnet_packet_task_cache, GFP_ATOMIC);
 	if (ptask == NULL)
 		goto fail;
@@ -1266,9 +1279,6 @@ static netdev_tx_t fwnet_tx(struct sk_buff *skb, struct net_device *net)
 	proto = hdr_buf.h_proto;
 	dg_size = skb->len;
 
-	/* serialize access to peer, including peer->datagram_label */
-	spin_lock_irqsave(&dev->lock, flags);
-
 	/*
 	 * Set the transmission type for the packet.  ARP packets and IP
 	 * broadcast packets are sent via GASP.
@@ -1290,7 +1300,7 @@ static netdev_tx_t fwnet_tx(struct sk_buff *skb, struct net_device *net)
 
 		peer = fwnet_peer_find_by_guid(dev, be64_to_cpu(guid));
 		if (!peer || peer->fifo == FWNET_NO_FIFO_ADDR)
-			goto fail_unlock;
+			goto fail;
 
 		generation         = peer->generation;
 		dest_node          = peer->node_id;
@@ -1344,7 +1354,8 @@ static netdev_tx_t fwnet_tx(struct sk_buff *skb, struct net_device *net)
 		max_payload += RFC2374_FRAG_HDR_SIZE;
 	}
 
-	dev->queued_datagrams++;
+	if (++dev->queued_datagrams == FWNET_MAX_QUEUED_DATAGRAMS)
+		netif_stop_queue(dev->netdev);
 
 	spin_unlock_irqrestore(&dev->lock, flags);
 
@@ -1355,9 +1366,9 @@ static netdev_tx_t fwnet_tx(struct sk_buff *skb, struct net_device *net)
 
 	return NETDEV_TX_OK;
 
- fail_unlock:
-	spin_unlock_irqrestore(&dev->lock, flags);
  fail:
+	spin_unlock_irqrestore(&dev->lock, flags);
+
 	if (ptask)
 		kmem_cache_free(fwnet_packet_task_cache, ptask);
 
@@ -1403,7 +1414,7 @@ static void fwnet_init_dev(struct net_device *net)
 	net->addr_len		= FWNET_ALEN;
 	net->hard_header_len	= FWNET_HLEN;
 	net->type		= ARPHRD_IEEE1394;
-	net->tx_queue_len	= 10;
+	net->tx_queue_len	= FWNET_TX_QUEUE_LEN;
 }
 
 /* caller must hold fwnet_device_mutex */